Portal type gantry cranes having long overhead girder frame spans are subject to considerable movement along the girder length and in the gantry leg due to picking up and releasing a load, moving the load along the girder, and moving the girder along the rails while carrying a load. This movement causes stressing of frame members which is very high at joints of braces in truss type gantry girders. This is especially true where the girders of portal cranes are cantilevered in the direction of the length of the girders beyond the legs. Where the trolley of the crane is moved to a position adjacent the legs including a position on the girder end cantilevered from the legs, the load on the adjacent leg is particularly high and the high load at the girder end causes cracking and failure of the girder brace connections.
When the loaded trolley is positioned adjacent a leg on either end of the girder frame, the concentration of the load on the leg highly loads the drive means which moves the leg along one of the rails on which the crane operates. The high loading of the leg may result in the drive means accelerating the highly loaded leg at a slower rate than the lightly loaded second leg of the crane is accelerated by its drive means. This results in a skew of the crane on the rails which significantly increases the stressing of the girder brace and leg joints. The additional stressing correspondingly greatly increases the brace joint cracking and failure.
There are various types of skew sensing systems for minimizing or preventing skew of a crane. However, these systems tend to be complex and expensive. The instant invention provides a relatively simple and inexpensive solution to skew problems.